1. Field of the Invention
The invention relates to drivers for ultrasonic transducers and to ultrasonic transducers. In particular the invention relates to a driver and ultrasonic transducer for use as a bonding apparatus in wire-bonding machines.
2. Description of Prior Art
Ultrasonic transducers on wire-bonding machines are used to apply pressure-and ultrasonic energy simultaneously for connecting the lead wires from a semiconductor component to a substrate. The ultrasonic transducer has an ultrasonic driver for converting driving signals from an ultrasonic signal generator into ultrasonic vibrations. A horn amplifies the ultrasonic vibrations from the driver and a bonding tool, which is normally in the form of a wedge or a capillary, located at the smaller end of the horn couples the amplified ultrasonic vibrations while applying a predetermined bond force to the lead wires and components being bonded together. Piezoelectric materials are used as the active (or driving) materials in the ultrasonic driver. It is difficult to induce electric polarization in thick piezoelectric elements and so a plurality of piezoelectric elements are stacked up to achieve the desired power level.
FIG. 1 shows a collapsed view and an exploded view of a prior art ultrasonic driver. The driver comprises a plurality of ring-shaped piezoelectric elements 1, 1′, 1″, 1′″ connected electrically in parallel and mechanically in series under a mechanical pressure exerted by a pre-stress mechanism formed by a metallic threaded shaft 2 and a metallic nut 3. Thin ring-shaped electrodes 4, 4′, 4″, 4′″, 4″″ are interposed between the piezoelectric elements 1, 1′, 1″, 1′″, shaft 2 and nut 3 to provide electrical interconnection and external-connection with an ultrasonic signal generator (not shown).
A problem with piezoelectric ultrasonic transducers is that they are subject to accelerated aging and fail when operating at elevated electrical, mechanical and/or temperature stress to increase mechanical power output. These transducers are not considered inherently reliable in high-power applications. Although multiple piezoelectric elements could be combined or grouped to produce transducers with increased power-handling capability this would result in inefficient transducers that are not only larger and heavier but also difficult to assemble and costly to operate.
Prior to the use of piezoelectric materials, magnetostrictive transition metals, such as iron (Fe), cobalt (Co) and nickel (Ni) were sometimes used. Although these early transition metals are less susceptible to deterioration under use, they are heavy and bulky. More importantly, their low transduction capability and low energy density significantly limit the mechanical power output from a given electromagnetic power input and volume of the material and so increase the size and weight of the resulting transducer.